In digital video systems, such as network camera monitoring systems, video sequences are compressed before transmission using various video encoding methods. In many digital video encoding systems, two main modes are used for compressing video frames of a sequence of video frames: intra mode and inter mode. In the intra mode, the luminance and chrominance channels are encoded by exploiting the spatial redundancy of the pixels in a given channel of a single frame via prediction, transform, and entropy coding. The encoded frames are referred to as I-frames. The inter mode instead exploits the temporal redundancy between separate frames, and relies on a motion-compensation prediction technique that predicts parts of a frame from one or more previously decoded frames by encoding the motion in pixels from one frame to another for selected blocks of pixels. The encoded frames may be referred to as P-frames (forward-predicted frames), which can refer to previous frames in decoding order, or B-frames (bi-directionally predicted frames), which can refer to two or more previously decoded frames, and can have any arbitrary display-order relationship of the frames used for the prediction.
At the site of reception of the encoded video sequence, the encoded frames are decoded. A concern in network camera monitoring systems is the available bandwidth for transmission of encoded video. This is particularly true in systems employing a large number of cameras. An analogous problem occurs regarding storage of images, for instance when storing images on an on-board SD card in the camera. A compromise has to be made, where available bandwidth is balanced against the interest of high quality images. A number of methods and systems have been used for controlling the encoding in order to reduce the bit rate of transmissions from the cameras. These known methods and systems generally apply a bit rate limit, and control the encoding such that the output bit rate from the cameras is always below the bit rate limit. In this way, it may be ensured that the available bandwidth is sufficient, such that all cameras in the system may transmit their video sequences to the site of reception, e.g., a control centre, where an operator may monitor video from the cameras of the system, and where video may be recorded for later use. However, applying a bit rate limit to all cameras may lead to undesirably low image quality at times, since the bit rate limit may require severe compression of images comprising a lot of details, regardless of what is happening in the monitored scene. For instance, it would be advantageous to get higher image quality when there is motion in the scene, e.g., in order to be able to identify an intruder, but a bit rate limit will instead lead to more severe compression, and thereby lower image quality, when there is motion in the captured scene.